Kyoudai no Rule.

Aiba Mikage is the older brother who likes "rules" and "order," and his brother a year younger is Tomoaki, who's loose and rough. They have contrasting personalities, but 12 years after their parent's remarriage, they have been getting along well. However, the winter Mikage's university entrance has been decided, their uncontrollable feelings break the "rules," and the two of them decide on new "rules" for themselves... Underneath their secret rules, the two of them desire each other strongly but their time to part is near. The erotic and dramatic love of step-brothers!

(Source: Senpai Notice Me Scans)